(;
AB[rk][pl][rl][qm][qn][qo][op][pp][nq][qq][rq][nr]
AW[rm][rn][ro][qp][rp][pq][pr]
C[Black plays.
Source: a classic problem from the centuries old Xuan Xuan QiJing collection.]
(;
B[sr]
C[A famous tesuji to create a dead corner shape. Amazingly, only this move works!];
W[qr]
C[White has no other choice. Otherwise, Black plays here to capture the stones at the bottom.];
B[rr]
C[Now Black almost has a bulky five shape in the corner. And just as bad, White has to make an approach move just to atari the corner stones.]
(;
W[rs];B[sm];W[qs];B[sn];W[ss];B[sq]
C[Black completes the bulky five. White dies.]
)
(;
W[sq]
C[White cannot even salvage an unfavorable multi-step ko with this throw-in.]
(;
B[so]
C[Black calmly makes a throw-in tesuji of his own. White is dead.];
W[sn]
C[For example ...];
B[sl];W[rs];B[sp];W[sq];B[sp]
)
(;
B[sm]
C[A mistake.];
W[so]
LB[rs:A]
C[Now White does have an unfavorable 2 step ko, although Black takes the ko 1st. (White has to make an approach move at A.)]
)
)
)
(;
B[rr]
C[Black's shape looks similar to the correct line, but ...];
W[sr]
C[... White takes the vital point with this sparkling tesuji.];
B[sq];W[rs]
C[Black has to fight a ko.];
B[ss]
C[For example ...];
W[qs]
)
(;
B[rs]
C[Black's corner shape looks good, but ...];
W[rr]
C[... now White has this corner tesuji.];
B[qr];W[sr];B[sq];W[ps]
C[You can easily verify that White wins the semeai by one move.]
)
(;
B[qr]
C[Also fails.];
W[rs]
C[Yet another must-know corner tesuji, which is to take the 2-1 point closest to the 2 weak stones.];
B[qs];W[ps]
C[White easily wins the semeai.]
)
)